1. Technical Field
Exemplary aspects of the present disclosure generally relate to a conveyor system that carries a sheet-type medium on a surface thereof and an image forming apparatus, such as a copier, a facsimile machine, or a printer including the conveyor system.
2. Description of the Related Art
There has been known a color image forming apparatus using an electrophotographic method in which toner images of different colors formed on latent image bearing members are primarily transferred onto an intermediate transfer member and then secondarily onto a sheet-type medium such as a recording medium in a secondary transfer process. There are two types of secondary transfer devices that performs the secondary transfer process employed in the image forming apparatus of this kind: a roller-transfer type and a belt-transfer type. The secondary transfer device of the roller-transfer type includes an intermediate transfer member and a transfer roller, and a sheet-type medium is interposed between the intermediate transfer member and the transfer roller, and is transported. The latent image is secondarily transferred onto the sheet-type medium while the sheet-type medium is transported.
The secondary transfer device of the belt-transfer type includes a conveyor belt (i.e., a secondary transfer belt) formed into an endless loop entrained about and stretched taut between support rollers. The sheet-type medium is interposed between the conveyor belt and the intermediate transfer member, and the latent image is secondarily transferred onto the sheet-type medium while the sheet-type medium is transported. In the secondary transfer device of the belt-transfer type, the sheet-type medium is interposed in a secondary transfer nip between the secondary transfer belt and the intermediate transfer member, and the sheet-type medium is absorbed to the secondary transfer belt upstream and/or downstream from the secondary transfer nip in the transport direction of the sheet-type medium. In this configuration, the sheet-type medium is held and transported reliably, not only at the secondary transfer nip, but also at the upstream side and the downstream side in the transport direction of the sheet-type medium. Thus, it is generally said that the belt-transfer type allows more reliable sheet conveyance than the roller-transfer type.
Similar to a generally-known belt conveyor, the belt transfer method may cause the secondary transfer belt to drift to one side in the width direction of the belt or repeatedly wander back and forth on either side in the width direction of the belt. Such belt wander and belt meander are attributed to dimensional tolerance of parts constituting the secondary transfer device, for example, variations in a parallelism error of rotary shafts of the plurality of rollers that supports the secondary transfer belt, variations in an outer diameter of the rollers, and variations in the tension of the secondary transfer belt due to changes in the circumferential length of the secondary transfer belt itself. More specifically, because of the reasons above, the secondary transfer belt does not travel linearly, but keeps traveling out of alignment in the width direction of the belt (i.e., the direction of the roller shaft), causing the belt to drift side to side.
In view of the above, various belt alignment devices that keep the belt on track have been proposed. One example of a known belt alignment device employs a shaft inclination method, in which a correction roller, around which the belt is entrained, capable of tilting, is employed to move the belt in the direction opposite the direction of the belt drift. However, the known belt alignment device of the shaft inclination method is disadvantageous when employed in a belt conveyor unit in which a sheet-type medium is carried successively on two or more conveyor belts arranged next to each other in the transport direction of the sheet-type medium.
For example, a sheet-type medium on a first conveyor belt disposed at the upstream side in the transport direction of the sheet-type medium is passed onto a second conveyor belt disposed downstream from the first conveyor belt. At this time, the leading end of the sheet-type medium separated from the surface of the first conveyor belt wound around a separation roller (support roller) disposed at the extreme downstream end in the transport direction of the sheet-type medium needs to land smoothly on the surface of the successive conveyor belt, that is, the second conveyor belt. If the leading end of the sheet-type medium does not land smoothly on the second conveyor belt, undesirable shock may be applied to the sheet-type medium, causing image failure on the sheet-type medium and paper jams, for example. Such difficulty becomes pronounced when using the belt alignment device of the shaft inclination method in which the degree of inclination of the separation roller is relatively large.
In view of the above, there is demand for an image forming apparatus capable of delivering smoothly the sheet-type medium from the first conveyor belt disposed at the upstream side in the transport direction of the sheet-type medium to the second conveyor belt disposed downstream from the first conveyor belt when using the belt alignment device of the shaft inclination method.